Fix strsep_quote() on strings without quotes.

For strings without quotes and escapes dstptr and srcptr are equal, so
zeroing *dstptr before checking *srcptr is not a good idea. In practice
it means that in -maproot=65534:65533 everything after the colon is lost.

The problem was there since r293305, but before r346976 it was covered by
improper strsep_quote() usage.

PR: 238725
MFC after: 3 days
Sponsored by: iXsystems, Inc.